The purpose of the proposed research is to investigate the biological mechanism causing the cognitive deficits that the applicant and others have documented in children treated early and continuously for phenylketonuria (PKU) and in children mildly hyperphenylalaninemic. They will also investigate whether the cognitive and neurochemical deficits are reversible. It is hypothesized that the deficits are caused by a mild reduction in the level of tyrosine reaching the brain, which selectively affects the dopaminergic projection to prefrontal cortex, impairing the cognitive functions dependent on prefrontal cortex. To test this, they will maintain the plasma phenylalanine (Phe) levels of experimental animals at the same level as has found been to be detrimental in PKU and hyperphe children (6-10 mg/dl; levels previously thought safe). Because the proposed hypothesis concerns the Phe:tyrosine ratio in blood, plasma levels of both amino acids will be measured. To test the hypothesis that cognitive functions dependent on prefrontal cortex are impaired, the mice will be tested on a task sensitive to prefrontal cortex function, delayed alternation. To test whether the mice are globally impaired, they will be tested on a control task, visual discrimination. The project will investigate the levels of dopamine, DOPAC, HVA, norepinephrine, MHPG, serotonin, 5-HIAA, Phe, tyrosine, and tryptophan in medial prefrontal cortex, anterior cingulate cortex, caudate-putamen, nucleus accumbens, entorhinal cortex, and hippocampal formation. Tyrosine levels will be measured to test the hypothesis that the problems are caused by lower levels of tyrosine reaching the brain. Tryptophan, serotonin, and 5-HIAA levels will be measured because tryptophan must also compete with Phe in crossing the blood-brain-barrier, so there is a risk of less tryptophan reaching the brain. Medial prefrontal cortex (homologous to dorsolateral prefrontal cortex in primates) and anterior cingulate are the 2 cortical areas richest in dopamine in the rodent brain. The caudate-putamen and nucleus accumbens are the subcortical areas richest in dopamine. The functions of prefrontal cortex and of the medial temporal lobe (entorhinal cortex and hippocampus) are intertwined in several respects, and performance on delayed alternation is sensitive to medial temporal lobe damage. It cannot be concluded that delayed alternation deficits are due to changes in prefrontal cortex without investigating other regions (e.g., medial temporal) that also participate in performance of the task. The level of hepatic phenylalanine hydroxylase activity will be empirically determined postmortem. Most animals will be bred from two homozygous parents; analysis of their DNA is unnecessary. Tissue from animals coming from mixed litter will be subjected to Western Blot analysis and RNase protection assay. The investigators will try to reverse the cognitive deficits and neurochemical effects by: (1) reducing dietary levels of Phe, and by providing supplementary (2) tyrosine, (3) tyrosine + tryptophan, and (4) L-dopa. These studies have direct implications for the treatment of phenylketonuric and hyperphenylalaninemic children, and for understanding the neural bases of cognition.